there will be stuff going on, on the weekend. the big clubs will be shut by then but there are alternatives. just a little early to tell what will be open etc. it‘s not 'summer ibiza' anymore though, for that you‘d need to arrive latest at the start of oct
Was there Last year at mid of october. If you know the Island from May to Sept. : Its totally different. Westend is a Ghost town , PdB nearly dead. Dunes was open , Nassau too , but less people. Nightlife : Additional Event in Heart at the Weekend.
There is a bus to Dalt Vila from San An. Go early and sit in the Streets. Petit Vermut for example or Croissant Show ( Closes at 11 ). Then you can easily walk along the Harbour to Pacha / Heart ( 10 -15 min). Out of the Season at the Weekends you will see residents and Born Ibiza People. Different to high Season.